So what is the guidline for crystal drops? I see that it looks like it's roughly a crystal per stradium (S6 - 5*, S5 - 4*, etc). But how else can I gain other level crystals? For instance I need four star crystals, but right now I can only find them in D19-21. If I play S6 on a lower difficulty (I always play on elite) will that mean that lower crystals will drop?

Thanks!